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Twitter users reacted fast to the explosions that ripped through the Boston Marathon Monday, but the incident also revealed how social media can only be so reliable in such situations.

Twitter spread news of the blasts quickly and was a useful communications tool for public authorities such as the Boston police and the marathon organizers. But information on social media sites can also be questionable or just plain inaccurate, noted Greg Sterling, senior analyst with Opus Research.

"It cuts both ways," Sterling said. "It allows you to get the information out more quickly, but it can also fan hysteria."

Two bombs exploded within 100 yards of each other near the marathon finish line on Monday afternoon. Police say two people were killed and dozens more injured. They have no suspects yet, and President Barack Obama has said it's not known yet if terrorists were involved.

The Boston Police Department's Twitter log showed a positive side of social media. It was updated minute by minute in the aftermath of the bombings, often with instructions about which areas to avoid, or information about where the most police officers might be stationed.

There was also misinformation, however. A report was circulated quickly on Twitter that police had shut down cellphone service in Boston to prevent detonation of further blasts, though it ultimately turned out to be inaccurate, according to network operators.

Others had nefarious intentions. At one point, a Twitter account with the handle @_BostonMarathon was promising to donate US$1 to victims of the blast for every one of its tweets that was retweeted. Users soon called it out as a fake, noting the real Twitter account for the Boston Marathon was @BostonMarathon.

That type of self-correction could be one of social media's strongest assets, said Karsten Weide, an analyst with IDC. There can be a lot of false or misleading content, but the nature of the service means that anyone, regardless of their credentials, can do some fact-checking.

Still, while Twitter is great at disseminating news fast, some see its value diminishing as time passes after an event. "Twitter does its best work in the first five minutes after a disaster, and its worst in the twelve hours after that," said one Twitter user, in a comment that was widely retweeted.

Twitter carried some graphic images of victims after the explosions, including blood-soaked sidewalks and people in the streets with severe injuries. One person urged users to focus on how to help rather than posting photos of victims.

Determining what's useful information and what crosses lines of decency or taste may come down to individual judgment, however. "There aren't really clear etiquette standards for using social media," said Sterling.

"As long as the event happens in a public space, there's no way to stop over-the-top or inappropriate information from getting out there," Weide said.

For sure, Twitter, Facebook and other social media sites were a useful source of information for many tracking the events. Google set up a Person Finder, as it did after the Japan earthquake two years ago, to help people connect with friends and loved ones after the incident.

Not surprisingly, the hashtag #bostonmarathon spiked sharply almost immediately after the attacks, said Hashtags.org, and mentions of "Boston" soared on Facebook, reported analytics company Topsy.